1. Field of the Invention
This invention relates to a ceramic sintered body useful as an insulated body in an insulated substrate in a circuit board for transmitting high frequency signals, which sintered body can be produced by firing at a low temperature, and has a low dielectric constant or a low dielectric loss in a high frequency region.
2. Description of the Prior Art
As ceramic wiring boards, those boards which are obtained by forming a wiring layer composed of high melting metals such as tungsten or molybdenum on the surface or in the inside portion of insulated substrates composed of ceramics such as alumina have been most spread.
With highly developed information in recent years, higher speeds were required in transmitting signals in semi-conductor elements mounted on a circuit board, and the integration of circuits and the density of the circuits were required to become high. Accordingly, the frequency bands of signals to be transmitted have become higher. However, conventionally known insulated substrates made of alumina ceramics are improper for trasmitting high frequency signals or high-speed transmission of signals. To trasmit signals at a high speed, the insulated substrate is required to have a low dielectric constant. However, the alumina ceramics have a high dielectric constant (9 to 9.5 at 3 GHz), and the dielectric loss in a high frequency such as microwaves or millimetric waves is high.
Very recently, there has been attracted an insulated substrate composed of a glass ceramic obtained by molding a mixture of glass and various inorganic fillers, and firing the mixture. This glass-ceramics has a low dielectric constant of about 3 to 7, and as compared with an alumina ceramic, is suitable for trasmitting high frequency signals. Furthermore, since the glass ceramics can be obtained by firing at a low temperature of 800 to 1000.degree. C., the above process also has an advantage that co-firing with a low resistant metal such as copper, gold or silver can be performed.
However, the conventionally known glass ceramic insulated substrate can be formed by co-firing with a low resistant metal such as copper, silver or gold, but has a defect of having a high dielectric loss. Since the dielectric loss at a frequency of a microwave region of at least 10 GHz is as high as at least 20.times.10.sup.-4, the insulated substrate does not have sufficient characteristics for use as a wiring board for transmitting high frequency signals.
Various electronic component parts are mounted on a multilayer wiring board and many input and output terminals are secured to the above multilayer wiring board, and furthermore, the multilayer wiring board is used by being connected to printed boards such as a mother board. Accordingly, this board should desirably have an approximate thermal expansion coefficient to the above electronic component parts, various terminals and the printed substrate. If the thermal expansion coefficients are greatly different from each other, when soldering is carried out at the time of providing an electronic component part or a terminal on the multilayer wiring board, or when soldering is carried out at the time of mounting the multilayer wiring board on the printed board, a large thermal stress occurs, and the thermal stress causes destruction of the multilayer wiring board, or causes disconnection in the wiring layer formed in the wiring board. Furthermore, the heat generated when the input or output of a signal into the wiring board is repeated leads to the same problem.
The wiring board formed by using the glass-ceramic insulated substrate has a problem that it is difficult to adjust the thermal expansion coefficient. In the above glass ceramic, it is difficult to adjust the thermal expansion coefficient by only adjusting a component which determines the dielectric characteristics. It is necessary to compound various thermal expansion coefficient adjusting agents in the glass ceramic. As a result, the dielectric characteristics of the glass-ceramic will be impaired.